Farm crops are continually plagued by a variety of pests which can stunt or damage crop growth or even completely destroy the crop. Some of the pests are in the form of weeds which grow similarly to the desired plant and compete for the nutrients provided by soil and water. Other pests are in the form of pathogens such as fungi and bacteria which are found in association with many plants.
One of the more serious problems associated with fungal pathogens in plants is root rot. For example, pea root rot caused by the fungus Aphanomyces euteiches is a serious problem in pea-growing areas, particularly in Wisconsin and other Great Lake states. The Aphanomyces fungus infects not only peas, but also snap beans and alfalfa, accounting for 10 to 15% losses in yield. In extreme cases, some fields, where the fungus population has been built up over the period of several years, have become essentially useless for these crops.
Despite efforts to develop fungicides and commercially acceptable pea cultivars with resistance to this pathogen, there is presently no commercially available product capable of controlling Aphanomyces. Currently, the best way to avoid the disease loss is to avoid planting susceptible crops in soils with a high population of the Aphanomyces fungus. Unfortunately, the fungus can survive for many years in field soil and a long rotational time to other crops is not practical. As a result, there is a need to find an alternative disease control strategy to eliminate root rot caused by Aphanomyces and possibly other fungi.
A second serious problem of fungal pathogens is blight, such as the leaf and stem blight of American and Chinese ginseng caused by the Alternaria panax fungus. The ginseng plant is a perennial grown from seed which takes at least 3 to 4 years to reach marketable size. Between growing seasons, the ginseng plants are covered with straw mulch on which Alternaria spores can survive. Although A. panax-induced leaf and stem blight typically does not kill the entire ginseng plant, it can lead to reduced ginseng root growth. To minimize Alternaria damage, ginseng growers currently treat their crops with an EBDC fungicide (mancozeb). Unfortunately, the fungicide breakdown products are highly carcinogenic and use of the fungicide is slated for elimination. At present, ginseng growers must obtain permission annually to use the fungicide. Yet, its use is necessary since it has been estimated that 80% to 100% of all ginseng plants would suffer Alternaria blight without effective fungal control.
Related Alternaria species similarly affect other plants, such as crucifers, potatoes, and tomatoes, and cause similar problems for commercial growers. In particular, early blight of potatoes caused by A. solani is an extremely serious problem for potato growers in humid climates. Likewise, in regions that grow plants for oils, such as rapeseed and canola, A. brassicae and A. brassicicola endemic.
There is increasing interest in the use of living organisms to control such root and foliar diseases. Microscopic organisms are present in soil in populations of approximately 1 billion per cubic inch of soil. Some of the microorganisms cause disease and some are beneficial. The beneficial microorganisms are of major interest. It has long been known in agriculture that certain of these microbial inoculants can be used to facilitate the growth of certain plant species or to assist the plants in suppressing particular pathogenic organisms. For example, it has been a common practice to inoculate soybeans and other legumes at planting with bacterial cultures of the genus Rhizobium so that nitrogen-fixing nodules will form as a result of the plant-bacterium symbiosis.
Reference is now made to U.S. Pat. No. 4,588,584 to Lumsden, et al. which discloses a particular species of Pseudomonas cepacia which is effective in controlling Pythium diseases of cucumber and peas. The term "biocontrol agent", as used herein, refers to a living organism which controls diseases.